ci: Fix merge_script in MSVC task #22912

The new merge_script in the MSVC build task does not really exit early when the task is triggered by a non-pr.

In the current code https://github.com/bitcoin/bitcoin/blob/e4aa9b15b9f80a08076ad329b473fe9107d9e65e/.cirrus.yml#L104

the exit 0 command exits from the PowerShell call, not the recent merge_script. This cause the next lines https://github.com/bitcoin/bitcoin/blob/e4aa9b15b9f80a08076ad329b473fe9107d9e65e/.cirrus.yml#L105-L107 are executed unconditionally.

Here is an excerpt from CI task log for the "Merge [#22915](/bitcoin-bitcoin/22915/): Remove confusing CAddrDB " commit:

...
C:\Users\ContainerAdministrator\AppData\Local\Temp\cirrus-ci-build>call powershell.exe -NoLogo -EncodedCommand ZwBpAHQAIAByAGUAcwBlAHQAIAAtAC0AaABhAHIAZAA= 
HEAD is now at 896649996 Merge bitcoin/bitcoin#22915: Remove confusing CAddrDB

C:\Users\ContainerAdministrator\AppData\Local\Temp\cirrus-ci-build>if 0 NEQ 0 exit /b 0 

C:\Users\ContainerAdministrator\AppData\Local\Temp\cirrus-ci-build>call powershell.exe -NoLogo -EncodedCommand aQBmACAAKAAkAGUAbgB2ADoAQwBJAFIAUgBVAFMAXwBQAFIAIAAtAGUAcQAgACQAbgB1AGwAbAApACAAewAgAGUAeABpAHQAIAAwADsAIAB9AA== 

C:\Users\ContainerAdministrator\AppData\Local\Temp\cirrus-ci-build>if 0 NEQ 0 exit /b 0 

C:\Users\ContainerAdministrator\AppData\Local\Temp\cirrus-ci-build>call powershell.exe -NoLogo -EncodedCommand ZwBpAHQAIABmAGUAdABjAGgAIAAkAGUAbgB2ADoAQwBJAFIAUgBVAFMAXwBSAEUAUABPAF8AQwBMAE8ATgBFAF8AVQBSAEwAIAAkAGUAbgB2ADoAQwBJAFIAUgBVAFMAXwBCAEEAUwBFAF8AQgBSAEEATgBDAEgA 
From https://github.com/bitcoin/bitcoin
 * branch                HEAD       -> FETCH_HEAD

C:\Users\ContainerAdministrator\AppData\Local\Temp\cirrus-ci-build>if 0 NEQ 0 exit /b 0 

C:\Users\ContainerAdministrator\AppData\Local\Temp\cirrus-ci-build>call powershell.exe -NoLogo -EncodedCommand ZwBpAHQAIABtAGUAcgBnAGUAIABGAEUAVABDAEgAXwBIAEUAQQBEAA== 
Already up to date.

C:\Users\ContainerAdministrator\AppData\Local\Temp\cirrus-ci-build>if 0 NEQ 0 exit /b 0 

This PR fixes this issue, and makes merge_script log more readable.

cc @MarcoFalke

Concept ACK f78cc90524dc15c8981da2a480621c2e47e3c7dd

I am not too familiar with ps so I can't review, but feel free to merge this if it seems right

I've updated PR description to elaborate the problem. @MarcoFalke

I am not too familiar with ps so I can't review, but feel free to merge this if it seems right

You could test the fixed behavior of the merge_script by running this branch on your personal Cirrus account. In that case a merging with the base branch must be skipped.

If you tested it in your fork, that should be sufficient. Feel free to merge.